Power Electronics

Monolithic 500-V, 1-A Three-Phase Motor Drive IC Fits SMT SOP

Manufactured using a SOI (silicon-on-insulator) process, an inverter IC consists of lateral IGBTs, HVICs, control and protection circuits all implemented on a single chip.

Motor drives for white goods such as dishwashers, air conditioners, and refrigerators have unique requirements compared to their industrial-environment counterparts. In particular, overall cost is an important factor, both for the motor drive itself as well as its integration into the specific appliance. Plus, home appliances are getting smarter so the drive needs to offer a simple easy interface with microcontrollers.

Typically, drives for high-volume production are manufactured on a highly automated production line. For white-goods applications, manufacturers prefer SMT components to minimize production costs. They also want ICs that require few external components. The Powerex M81500FP is a 500-V, 1-A motor drive that meets the requirements for home-appliance applications with ratings up to 100 W.

PERFORMANCE AND FEATURES

The M81500FP motor drive employs a 33-pin SOP package (Fig. 1). Some pins of the lead frame are connected to each other, which improves the heat transfer from the lead frame to the PCB. In addition, the pin pitch is different for the control and power part of the IC, reflecting the various creepage and clearance requirements of the high- and low-voltage sides of the package.

The M81500FP features a lead frame construction and a single silicon chip with IGBTs provides control, drive and protection functionality. Free-wheeling diodes and bootstrap diodes are external.

The silicon chip is mounted upside down in the package to optimize the thermal impedance from the junction to the surface of the power semiconductors. The lead frame, via its heat-spreading function, is efficiently used to enlarge the thermally active surface of the SOP package, thus reducing the junction-to-ambient thermal resistance.

Besides the thermal interface to the PCB, this construction detail offers a second, electrically isolated and very efficient way of cooling the single-chip inverter IC. The combined effect of the dual thermal paths provides a junction-to-case thermal resistance of about 5°C/W, and when mounted to a typical FR4 PCB, a total junction-to-ambient thermal resistance of 48°C/W in still air is offered.

Fig. 2 is a block diagram of the M81500FP. The V cc terminal supplies a single 15-V source to the IC. The UPIN, VPIN, UNIN and WNIN inputs are compatible with 3-V to 5-V logic, so they can be controlled directly by DSPs or microcontrollers. Internal threshold voltages are derived from a stable internal reference voltage, VREG. The logic-input signals pass through the input control to prevent shoot-through errors caused by the simultaneous in-phase turn-on of the N-side and P-side inputs.

Boot-strap techniques provide floating power supplies for the P-side IGBT gate drive, derived from the 15-V source connected at the Vcc supply pin. The M81500FP has built-in boot-strap diodes to simplify implementation of these circuits.

CIRCUIT PROTECTION

Undervoltage (UV) detection circuits protect the N-side and P-side against operation at dangerous low voltages. If any of the power supplies falls to an unsafe level the corresponding section of IGBTs is turned off; and in the case of an N-side undervoltage situation, the fault output is activated to indicate the dangerous condition.

The IGBTs in the M81500FP are protected against short-circuit conditions by a built-in high-speed comparator that detects a voltage of 0.5 V to shut off the IGBTs. Such a voltage is derived from a shunt resistor connected between VNO1, VNO2 and GND pins, setting the overcurrent protection level. Also, this inverter contains a thermal shutdown that operates at approximately 140°C and has a hysteresis of about 20°C. The thermal-shutdown function blocks IGBT switching and issues a fault signal.

An internal, blanking, first-order low-pass circuit with a 0.5-µs time-constant connects to the “CIN” terminal, which eliminates transient currents like leading edges of recovering free-wheeling diodes. This simplifies the operation of the M81500FP and also reduces the number of externally required components. The minimum number of necessary passive components to operate the inverter IC is three ceramic bootstrap capacitors, one 1206-class shunt resistor and a ceramic capacitor at the Vcc pin.

The inverter's silicon chip employs Silicon On Insulator (SOI) technology produced by a 1.3-µm HVIC process. The power stage consists of HV diodes and an N-channel lateral IGBT having a cylindrical structure for high short-circuit robustness and low Vce(sat).

The control section uses 24-V CMOS technology, whereas the level shifters use a HV-NMOS structure. Safe and rugged operation requires isolation techniques, both for the surface and laterally. High-voltage trench isolation is obtained by a Resurf technique while a multiple floating field plate (MFFP) stabilizes the surface electrical field. These technologies control the electrical field strength during transient operation (switching) dynamically and also for steady-state operation. Both technologies have been successfully applied already to previously developed 600-V and 1200-V high-voltage ICs (HVIC) and have proven reliable.

OPERATING WAVEFORMS

Typical operating waveforms for the turn-on and the turn-off are shown in Fig. 3. The turn-on waveform of the IGBT exhibits a desirable softly declining waveform at 125°C where IC= 0.5 A, and the maximum dV/dt is only 1.7 kV/µs. The turn-off behavior at a dc-link voltage of 300 V shows a small voltage overshoot and moderate di/dt, indicating that the IGBT is optimized for low noise.

Short-circuit withstand capability is an important feature to ensure highly reliable operation of the drive system. Hence, the IGBT must withstand the high stress during a short circuit until the built-in short-circuit detection and protection function activates and shuts down the IGBTs. In today's state-of-the-art transfer mold IPMs, a typical short-circuit withstand capability of about 4.5-µs of the IGBT chip using 0.6-µm planar technology or 1-µm Carrier Stored Trench Bipolar Transistor (CSTBT) was realized and the control IC was able to turn off the device within a microsecond.

Fig. 4 shows the actual capability of the lateral IGBT, in which the device survives a 21-µs short circuit, when the starting temperature of the short circuit is 25°C. The more severe case for the IGBT is when the short circuit occurs while the junction temperature is already on a high level, e.g. when the maximum load is applied to the IC just before the fault. Using a starting junction temperature of Tj =125°C, the short-circuit withstand capability was shown to be approximately 10-µs. In addition, the clean interface layout and separation of the power section from the control signal lines on the IC makes it possible to route the complete circuit on a single PCB layer.

PACKAGING THE INVERTER

The inverter is housed in a transfer-mold package. Well-suited for large-scale IGBT module and IPM (intelligent power module) production, transfer-molding technology has been widely used to manufacture power modules with ratings from a few hundred watts to more than 4 kW.

The main advantage over competing packaging technologies is the dual use of the copper lead-frame as an electrical conductor as well as a heat spreader. Furthermore, this technology can use bare IGBT and control ICs instead of mounting pre-packaged components.

This technology has been improved particularly with regard to the thermal resistance of the module and the maximum achievable output power while new packages have been introduced to respond to further miniaturization trends.

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